This invention relates to the production of polyurethane foam and more particularly to a method and apparatus for enhancing the mixing of the liquid polyurethane foam components resulting in a foam having a fine cell structure.
Various processes for producing polyurethane foam are known in the art. In general, a polyol component and a isocyanate component are reacted together in the presence of a catalyst, a surfactant and a chemical blowing agent such as a fluorocarbon or a chlorofluorocarbon (i.e. Freon.RTM.). Certain fluorocarbons and chlorofluorocarbons, however, have been determined to be pollutants of the atmosphere and thus the dependence thereon is undesirable.
An alternative to the use of these compounds has been suggested and comprises the mechanical induction into the polyol or isocyanate compounds of a gas such as air, carbon dioxide or nitrogen, which does not detrimentally effect the atmosphere. For example, U.S. Pat. No. 3,862,879 discloses a process wherein the gas and all of the urethane components, except for the catalyst, are fed into a mixer, the components are frothed, the catalyst is mixed with the froth and the froth is directed into a mold.
Another previously suggested alternative is to use a reaction injection molding (RIM) system wherein the polyurethane forming components are mixed by high pressure impingement of the components together and injection of the mixture into a heated mold. The foaming for the RIM system may be facilitated by entraining gas bubbles in the mixture to lower its viscosity such as described, for example, in U.S. Pat. No. 3,882,052 to Raynor et al or U.S. Pat. No. 4,157,427 to Ferber.
The limitations of existing processes and apparatus, however, are that it is difficult to obtain uniform mixing and a uniform cellular structure without utilizing complicated processes and cumbersome apparatus. Moreover, these processes and apparatus often result in products having an undesirable coarse cell structure and the time of reaction of the processes is too slow. These drawbacks are particularly a problem when making foam packaging materials and foam packaging cushions which require a fine cell structure in order to have optimum cushioning.
It is accordingly an object of this invention to provide an improved method and apparatus useful for producing polyurethane foams having a fine cell structure.
It is a feature of this invention to provide enhanced mixing of the reaction components of the polyurethane foam which results in the desired fine cell structure.
It is an advantage of this invention to eliminate the dependence on the use of fluorocarbons and chlorofluorocarbons by utilizing a non-condensible non-reactive gas.